In the present specification, reference is made to the following publications cited for illustrating prior art techniques, in particular with regard to generating fs laser pulses by Kerr lens mode-locking:    [1] D. E. Spence et al. in “Opt. Lett. 16, 42-44 (1991);    [2] U. Keller et al. in “Selected Topics in Quantum Electronics, IEEE Journal of 2, 435-453 (1996);    [3] P. F. Moulton et al. in “J. Opt. Soc. Am. B 3, 125-133 (1986);    [4] R. Ell et al. in “Opt. Lett. 26, 373-375 (2001);    [5] M. Tokurakawa et al. in “2008 Conference on Lasers and Electro-Optics & Quantum Electronics and Laser Science Conference, Vols 1-9, 200-201 (2008);    [6] S. Matsubara et al. in “Frontiers in Optics 2008/Laser Science XXIV/Plasmonics and Metamaterials/Optical Fabrication and Testing, OSA Technical Digest (CD) (Optical Society of America, 2008), FTuT1;    [7] S. Matsubara et al. in “Conference on Lasers and Electro-Optics 2010, OSA Technical Digest (CD) (Optical Society of America, 2010), CTuV2;    [8] Y. Sasatani et al. in “International Journal of Latest Research in Science and Technology 1, 2 (2012);    [9] M. Tokurakawa et al. in “Opt. Lett.” vol. 33, p. 1380-1382 (2008);    [10] F. Brunner et al. in “Opt. Lett. 27, 1162-1164 (2002);    [11] U. Sadao et al. in “Jpn. J. Appl. Phys. 50, 010201 (2011);    [12] H. A. Haus et al. in “J. Opt. Soc. Am. B 8, 2068-2076 (1991);    [13] V. L. Kalashnikov in “Solid State Laser, P. A. Al-Khursan, ed. (InTech, 2012);    [14] T. Sudmeyer et al. in “Nature Photon.” 2, 599-604 (2008);    [15] C. Schriber et al. in “Opt. Express 22, 18979-18986 (2014);    [16] K. F. Mak et al. in “Opt. Lett.” 40, 1238, (2015);    [17] O. Pronin et al. in “Nat. Commun.” Acc. (2015);    [18] J. Brons et al. in “Opt. Lett.” 39, 6442-6445 (2014);
and    [19] C. Radzewicz et al. in “Optics Communications”, vol. 102, p. 464-468 (1993).
Current applications of laser pulses often require a short pulse duration (broad spectrum). For instance, XUV generation is more efficient when the pulse duration approaches the sub-50 fs regime. Difference frequency generation (DFG) is also desirably performed with a broad spectrum in order to be able to reach broad spectrum in the mid-infrared range. The applications in multiphoton-microscopy, optical coherence tomography also benefit from the short pulses.
The field of ultrafast laser pulse sources (laser oscillators) has experienced dramatic changes over the last two decades. This was mainly due to the development of the mode-locking methods like Kerr-lens mode-locking (KLM) [1] and mode-locking using semiconductor saturable absorber mirrors (SESAM) [2] and their implications in the crystalline solid-state and fiber oscillators [14]. The availability of the relatively cheap and bright diode lasers has prompted the development in Yb, Er, Ho, Tm-doped oscillators (and many other materials) which can be pumped by those diode-lasers. Numerous attempts were made to develop Yb doped materials of a good quality and being able to support pulse durations of about 100 fs (see references in [15]). Many experiments with those materials were concentrated on achieving shortest pulse duration directly from the oscillators. Alternative approaches rely on external spectral broadening and pulse compression performed in solid-core and gas-filled fibers [16] and recently in crystals [17].
There are several important factors which play a crucial role in achieving shortest pulses directly from an oscillator:
a) emission bandwidth of the gain profile of the gain medium,
b) mode-locking technique and its self-amplitude modulation coefficient, relaxation time and starting capability, and
c) dispersion compensation and management (this is especially critical when the pulse duration approaches the few-cycle regime).
A broad emission spectrum of the gain medium supports short pulses. An excellent example of an extremely broadband gain medium is Ti-doped Sapphire (Ti:Sa) [3]. FIG. 7 (prior art) illustrates a conventional laser pulse source apparatus 100′ as described in [4], which created the shortest pulse duration ever directly achieved from a Ti:Sa oscillator. The laser pulse source apparatus 100′ comprises a resonator cavity 10′ with a Z-configuration, including the Ti:Sa crystal as a gain medium 21′ and Kerr-medium and a BK7 glass plate as additional Kerr medium 31′. However, while the emission spectrum of the Ti:Sa crystal should theoretically support pulses with a duration of about 3 fs, the pulse duration obtained with this setup was 4.8 fs. In other words, the emission bandwidth limit was not reached in [4].
On the other hand, it is possible to generate pulses with a spectrum exceeding the emission bandwidth of the gain medium [5-8]. This fact does not contradict theory and relies on the utilization of nonlinear effects inside of the resonator cavity and soliton mode-locking with rather high coefficients of self-amplitude modulation (SAM). Thus, a spectrum with the full width half maximum (FWHM) of 18.9 nm was generated in a standard Z shape cavity combining both KLM and SESAM techniques [5] for the gain medium Yb:Lu2O3 which supports only about 13 nm FWHM of the emission profile. The combination of KLM and a SESAM effectively increases the SAM coefficient and leads to a broader spectrum. A similar resonator geometry and the combination of KLM and a SESAM allowed for the generation of 131 fs with a spectral FWHM of 12 nm [8] or an even broader spectrum [6, 9] from an Yb:YAG gain medium (supporting only 9 nm emission spectrum).
However, the SESAM based mode-locking has several disadvantages. Firstly, due to the limited overall bandwidth, the pulse durations obtained with the SESAM techniques are relatively large, e.g. in a range of about 100 fs. Furthermore, SESAM elements are quite expensive, strongly wavelength dependent, exhibit early onset of two-photon absorption, have non-saturable losses and rather low damage threshold. The saturation dynamics of SESAMs leads to additional instabilities as onset of Q-switching. Those critical issues prevent operation at high pulse intensities and average powers.
Mode-locking further can be influenced by spectral filtering as demonstrated in [10, 11]. In the first case [10] filtering is realized via the combination of a prism and a knife edge. In [11], spectral filtering is realized via dielectric mirrors which suppress the gain at its maximum around 1030 nm and also strong reabsorption in a highly-doped Yb:YAG crystal.
Soliton dynamics (mode-locking) is described by complex nonlinear Ginzburg-Landau equation [12, 13]. Depending on the equation simplification used to approximate soliton mode-locking regime different models can lead to the qualitatively different results. For instance, one can completely neglect the self-amplitude modulation (SAM) mechanism or assume perfectly saturable, fast or slow SAM mechanism. However, different models show that SAM coefficient (modulation depth) and its saturation behavior play important role in the mode-locking. For instance, taking the model used in [12] with fast saturable absorber leads to the dependency
  τ  ∼      1                  Δ        ⁢                                  ⁢        R            where τ is the pulse duration and ΔR is SAM coefficient (or:modulation depth). Thus increase in the modulation depth leads to the decrease in the pulse duration. This influence is significant and cannot be neglected.
Due to the shorter pulse durations, there is a particular interest in the KLM-based creation of laser pulses, which however has a limitation due to a pulse intensity dependent saturation and roll-off of the SAM mechanism as described in [13]. FIG. 4 of [13] describes the SAM behavior with a single Kerr medium inside of the cavity. Qualitatively, an increase of intra-cavity power leads to saturation and then roll-off of the power-dependent and artificial intra-cavity losses.
Finally, ultrafast laser pulses can be created by additional spectral broadening and compression, which increase complexity, size and overall costs of the system in conventional laser pulse sources. Furthermore, it may result in additional intensity noise and timing jitter. Applications requiring very clean pedestal-free pulses suffer from the typically bad pulse compression quality with strong pedestal pulses carrying a substantial amount of energy (sometimes up to 50%).
Publication [19] discloses the use of monocrystalline ZnS as an additional highly nonlinear intracavity self-focusing element in a Ti:sapphire self-mode locked laser. A quartz oscillating Brewster plate (OBP) is included in the laser set up, which is used to start mode locking by mechanically oscillating the incidence angle around the Brewster angle only. This mechanical oscillation introduces noise and helps to start mode-locking. When the mode locking is broken, the OBP is used to restore mode locking. According to [19], only the ZnS acts as the additional highly nonlinear intracavity self-focusing element, while there is no idea of using the OBP for self-amplitude modulation or self-focusing.